Sarah Huckabee Sanders, the White House press secretary, announced the decision on Twitter on Thursday morning, saying that President Trump had authorized it after a request from Gov. Ricardo A. Rosselló of Puerto Rico.

At @ricardorossello request, @POTUS has authorized the Jones Act be waived for Puerto Rico. It will go into effect immediately.

“It is an act of justice. It will allow Puerto Ricans to rebuild and to have a cost of living that really frankly is affordable,” Carmen Yulín Cruz, the mayor of San Juan, said on CNN on Thursday.

Several members of Congress had requested the waiver earlier in the week, saying it would facilitate delivery of food, medicine, clothing and other supplies to the island, which was devastated by Hurricane Maria.

The decision was made by Elaine Duke, the acting head of Homeland Security, and comes after Defense Secretary Jim Mattis determined that doing so would be in the interest of national defense, according to a Homeland Security spokesman. The waiver will be in effect for 10 days.

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Workers unload shipping containers at the port in San Juan.CreditAlvin Baez/Reuters

Here’s a brief guide to the law, why Puerto Rico wanted it waived, and where the disaster relief effort stands.

What is the state of disaster relief in Puerto Rico?

Governor Rosselló and federal officials on Thursday said that more fuel, food and flights were beginning to reach the island. Increasingly desperate people have spent hours lining up outside grocery stores, banks and gas stations, only to sometimes leave with nothing.

Amid widespread shortages of basic necessities, Mr. Rosselló and federal officials said they were setting up 11 regional centers to distribute aid to devastated towns that were all but cut off from the outside.

Federal officials said these centers had received two million liters of water and almost one million meals, and more was in the pipeline. A main challenge has been getting supplies out of Puerto Rico’s ports and to people who need food and water.

Governor Rosselló said that 689 of Puerto Rico’s 1,100 gas stations were now open, but lines at many still stretch for a quarter-mile and people spend entire days to fill up a few plastic jugs with fuel for their cars or generators. He said that more fuel was arriving, and that officials were working to get truckers to transport fuel and other supplies.

What is the Jones Act?

The Merchant Marine Act of 1920, known as the Jones Act, requires goods shipped between points in the United States to be carried by vessels built, owned and (mostly) operated by Americans.

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Hurricane Maria caused widespread damage across Puerto Rico, and officials said the Jones Act was hindering efforts to get supplies to the island.CreditJoe Raedle/Getty Images

Its goals were twofold. First, it was intended to support a national maritime industry that could mobilize for war or a national emergency. Second, it was intended to protect American control over local waterborne commerce.

Those opposed to the act have long included officials and allies of Hawaii, Alaska and Puerto Rico, who say that the law increases shipping costs for goods from the mainland, which are then passed on to consumers. Its supporters include pro-defense groups and members of the domestic shipping industry whose interests it protects.

Some criticized calls for the suspension, arguing that it was not clear how much good it would do and that delivery delays were being caused by problems moving goods inland, not importing them.

Why did Puerto Ricans want a waiver?

In a Monday letter to the head of the Department of Homeland Security, eight members of Congress asked for a temporary waiver of the act, arguing that lifting it would expand access to food, medicine, clothing, building supplies and oil needed for power plants.

“Puerto Rico can’t borrow funds and they are required to use American shipping only, which is the most expensive in the world. In their hour of need, Washington can help by suspending the Jones Act,” one of the letter’s signatories, Representative Luis V. Gutiérrez, Democrat of Illinois, said in a statement at the time.

A day later, Senator John McCain, Republican of Arizona, voiced his support for the effort.

“It is unacceptable to force the people of Puerto Rico to pay at least twice as much for food, clean drinking water, supplies and infrastructure due to Jones Act requirements as they work to recover from this disaster,” he said on Tuesday.

Mr. McCain, long an advocate for repealing the law, most recently advanced legislation to do so in July. He renewed his call for repeal on Thursday after the waiver was announced.

How are waivers granted?

Waivers are issued by the Department of Homeland Security, typically at the request of shippers unable to find United States vessels to shuttle their cargo or at the request of other federal agencies.

A waiver can be granted only if it is in the interest of national defense and only if there are enough United States vessels available to meet national defense needs. Shipping costs or humanitarian needs are not part of the consideration, officials said.

Alternatively, a request in the interest of national security may be granted automatically if it comes from the secretary of defense.

When was the last waiver?

The Jones Act was most recently waived weeks ago, in response to hurricanes Harvey and Irma.

That decision, she said, would make it easier to ship petroleum products, including gas, diesel and jet fuel, from New York, Louisiana, Pennsylvania and Texas, to Georgia, Florida, South Carolina and Puerto Rico. The one-week waiver was ultimately extended through Sept. 22.

That waiver was granted about two weeks after Harvey first made landfall and just days after Hurricane Irma clobbered the Caribbean. Thursday’s waiver was issued eight days after Hurricane Maria first made landfall in Puerto Rico.

Before that, a waiver had not been issued since December 2012, when one was granted to ease the delivery of petroleum products after Hurricane Sandy, according to the department.